Mayne Pharma files damages claim against Cosette

Mayne Pharma Group (ASX:MYX) has escalated its legal dispute with Cosette Pharmaceuticals, filing a substantial damages claim in the Supreme Court of New South Wales.

The lawsuit follows the December 2025 termination of a scheme implementation deed that would have seen Cosette acquire the Australian-based pharmaceutical company.

The legal action targets not only Cosette Pharmaceuticals for an alleged breach of the agreement but also extends to Avista Capital and its CEO, David Burgstahler, whom Mayne Pharma accuses of inducing that breach.

Seeking compensation on behalf of both the company and its shareholders, Mayne Pharma’s board stated it is committed to pursuing "substantial damages" to address the fallout of the failed deal.

The move signals a high-stakes confrontation between the specialty pharma leader and its private equity-backed counterparts as the market awaits further disclosures on the progression of the case.

At the time of reporting, Mayne Pharma's share price was $2.65.